In the field of micro-processing typified by the manufacture of an integrated circuit element, a lithography technique is required which makes it possible to realize more finely processing for a level to 0.10 μm or smaller. In the conventional lithography process, near ultraviolet rays such as i-rays are applied as radiation, however, it is said that micro-processing for a level to 0.10 μm or smaller, that is a level to subquarter micron is extremely difficult when the near ultraviolet rays are applied. Accordingly, the lithography using radiation having a shorter wave length than the near ultraviolet rays has been studied to enable micro-processing for a level to 0.10 μm or smaller. The short wave length radiation may be far ultraviolet rays including bright line spectrum by mercury lamp and excimer laser, X rays, electron beams, or the like. Among these, KrF excimer laser (wavelength 248 nm), and ArF excimer laser (wavelength 193 nm) are of particular interest.
Accompanying the interest of excimer laser, a number of resist film materials for excimer laser is proposed. An example is a composition (hereinafter, referred to as “chemically-amplified resist”) utilizing the chemical amplification effect based on a component having an acid dissociable functional group and a component (hereinafter, referred to as “acid generator”) which generates an acid upon being exposed to radiation (hereinafter, referred to as “exposure”). A composition has been proposed as the chemically-amplified resist, which comprises a resin having a t-butyl ester group of a carboxylic acid or t-butyl carbonate group of phenol and an acid generator. Regarding the composition, the t-butyl ester group or t-butyl carbonate group in the resin dissociates by an action of an acid generated upon exposure, whereby the resist has an acidic group such as a carboxyl group or a phenolic hydroxyl group. As a result, the exposed areas on the resist film become readily soluble in an alkaline developer and a desirable resist pattern can be obtained.
Formation of finer patterns (a fine resist pattern with a line width of about 45 nm, for example) will be required for such a micro-processing. Reducing the wavelength of a light source of an exposure apparatus and increasing the numerical aperture (NA) of a lens are thought to be a solution for forming a finer pattern. However, the reduction of the wavelength of a light source requires a new exposure apparatus and the apparatus is expensive. In addition, increasing the NA of a lens involves a problem of decreasing the depth of focus even if a resolution is increased due to a trade-off relationship between the resolution and the depth of focus.
Recently, a liquid immersion exposure process (i.e., liquid immersion lithography) has been reported as a lithography technique enabling a solution to such a problem. In the liquid immersion exposure process, a liquid refractive-index medium (liquid for the liquid immersion exposure process) such as pure water or a fluorine-containing inert liquid, which has a predetermined thickness, is interposed between a lens and a resist film formed on a substrate, that is, on the surface of the resist film. In this method, air or an inert gas such as nitrogen which has been conventionally used in an exposure optical path space is replaced with a liquid for immersion exposure having a larger refractive index (n) than air. Therefore, if the conventional light source is used, the same effect can be obtained as the case in which a shorter wavelength is used. That is to say, the resolution can be increased without decreasing the depth of focus.
Accordingly, since a resist pattern having a higher resolution and excellent depth of focus can be formed at a low cost using the lens mounted on the existing apparatuses by utilizing the liquid immersion exposure process. And a number of compositions for liquid immersion exposure are reported in, for example, Patent Documents 1 to 3.